Butyl rubber was one of the earlier synthetic rubbers to be developed. It is a copolymer of isobutylene and a conjugated multiolefin, usually isoprene. It was long recognized that the isoprene could not be recycled in the butyl rubber process because the recycled monomer stream acted as a poison to the polymerization process. It was not until after a substantial number of years of discarding isoprene that the cause of this poisoning effect was understood. It is now known that the poisoning effect is the result of the formation of t-butyl chloride (t-BCl) from isobutylene reactions with the HCl. The t-butyl chloride tends to concentrate by fractionation in the recycle isoprene stream and affects the polymerization by chain transfer mechanisms so that lower molecular weight copolymers are produced.
Since t-butyl chloride and isoprene cannot easily be separated from one another by standard fractionation techniques, butyl rubber plants continue to discard the unreacted isoprene which has been used in the butyl rubber process rather than recycling it. The result is a significant increase in polymerization costs. For example, in a 50,000 ton/yr. butyl rubber plant, isoprene recovery would reduce isoprene raw material costs by about $600,000 per year.
Many techniques are known for removing halides from process streams. For example, in the production of methyl chloride and methylene chloride by the (oxy) chlorination of methane, the chlorides can be recovered by gas phase adsorption in beds of adsorbed materials including silica gels, activated carbon, activated alumina, molecular sieves, or their combinations; see U.S. Pat. No. 4,020,117. The adsorption is carried out at about -50.degree. C. to about 20.degree. C. The adsorbed halides are stripped from the adsorption stage at about 100.degree.-400.degree. C. Similarly, German Patent No. 2,839.5165 discloses a process for purifying an exhaust gas stream to remove contaminants such as halogens or halogenated hydrocarbons by passing the gas through alumina or calcium compounds.
British Patent No. 1,438,246 discloses a process for reacting a chloroform process stream containing impurities by contacting the stream in the vapor phase with activated carbon or alumina. It is alleged that CH.sub.2 ClBr which is present as an impurity in the chloroform, reacts with the chloroform to form CHCl.sub.2 Br and CH.sub.2 Cl.sub.2, which are then readily separated from the chloroform by distillation.
Soviet Union Patent No. 506,597 teaches the purification of recycled methylene chloride-isobutylene stream by passing the compounds first in the vapor phase over alumina and then in the liquid phase at 10.degree. to 20.degree. C. It is disclosed that the process removes water, dimethyl ether, and HCl from the stream.
U.S. Pat. No. 2,347,945 discloses a method for removing organic fluorides from a hydrocarbon stream either in the liquid or gaseous phase by contacting the stream with a "contact material." The contact material can be alumina, hydrated bauxite, chromium oxide, and metals from the iron groups, especially nickel deposited on an inert support.
U.S. Pat. No. 3,864,243 discloses a process for the removal of combined chlorine (organic or inorganic) from a hydrocarbon stream by percolating the hydrocarbon through a bed of dehydrated activated alumina, e.g. bauxite. The adsorption process is said to be more effective at room temperature than at elevated temperatures, e.g., 98.degree. C. Similarly, U.S. Pat. No. 3,862,900 discloses the room temperature adsorption of organic halides on molecular sieves (pore size 7-11 A).
U.S. Pat. No. 2,412,220 discloses a process for the removal or organic fluorides from a hydrocarbon stream by passing the hydrocarbon through a bed of alumina which is catalytically active for hydrogenation or dehydrogenation. It is alleged that the effluent stream contains silicon fluorides which are subsequently removed by treating the hydrocarbon stream with an alkali metal hydroxide, e.g., NaOH, and then filtering the hydrocarbon stream, through a non-siliceous granular filter medium, e.g., charcoal. In a similar vein, U.S. Pat. No. 2,391,149 discloses the removal of fluorides from a hydrocarbon stream by contacting the hydrocarbon with alumina which has been impregnated with an alkali metal hydroxide.
While the art generally teaches the use of materials such as activated carbon and alumina for the purification of halide containing process streams, it is apparent from these disclosures that not all organic halides are removed from a process stream contacted with these and other materials of the prior art. Furthermore, there is no disclosure of the removal of t-butyl chloride from such hydrocarbon streams; nor is there any teaching from which it could be concluded that a particular contact medium is preferred over others for the removal of t-butyl chloride from an isoprene stream.